3/8  bolts 3 or 3 and a quarter long should do it . I use regular hex head bolts .  And regular size washers . Put  the bolts up through the bottom . If the head is  not beat up or bent only a thread or two will stick out of the lock nut . I never had good luck using carriage bolts . If you put carriage bolts down through  , the threads get mangled on scraping the dirt . Regular bolts up through have not caused any issues with feeding at the auger . And they are a lot easier to remove then carriage bolts . 
As to  what every one else said is correct  about holding the skids in place .
If your cutter bar is not level after bolting it up . Adjust the leveling bolts at the rear . Most times  I dont need to tighten the 3/8 bolts extremely tight . That is why I use lock nuts .